1. Field of the Preferred Embodiment(s)
This invention generally relates to the manufacture and testing of resistors. Specifically, there is a test system for z-axis strain gauge resistors on a ceramic substrate used in pointing devices for controlling the positioning, movement and operation of a cursor on a display screen. The z-axis resistors act as the activation button for selecting items on the display screen by tapping on the pointing stick instead of clicking on a mouse button.
2. Description of the Related Art
Various devices are well known for controlling cursor movement over a computer display screen of a computer and for signaling a choice of computer command identified by the position of the cursor on the display screen menu.
Manufacturers of portable laptop computers, recognizing the need for placing the cursor controlling device in a permanent and more convenient location, installed a small stubby, button-like joystick centrally around the keys of the computer keyboard, specifically at the juncture of the "g," "h" and "b" keys of the standard "QWERTY" keyboard. The joystick, also known as a pointing stick, was sensitive to lateral pressure, the amount and direction of which were sensed and input into the computer to cause movement of the cursor, and the speed and direction of cursor movement corresponded to the amount and direction of pressure on the joystick. However, the manufacturer has to provide upwardly extending "mouse" or "click" buttons somewhere on the computer.
It is desirable to be able to allow the user to both control the cursor movement and select items on the display using exclusively a pointing stick device. In other words, to allow the user dual control by using only one finger while allowing the remaining fingers to reside on the home row of the standard keyboard.
An exemplary design for a pointing stick is detailed in pending patent application Ser. No. 08/756202 entitled, "Z-axis sensing pointing stick with base as strain concentrator", owned by the same assignee of record of the instant application and is herein incorporated by reference. The 08/756202 application shows details in FIG. 1 of a z-axis pointing stick with a ceramic substrate having strain gauge resistors on all four sides.
The resistors have a voltage and current applied to it during operation supplied from associated electronic circuits on the computer or other device. The four resistors for sensing X and Y cursor movement are arranged in two half wheatstone bridges. The balanced bridge serves to keep the total resistance constant between the applied voltage and ground during X and Y cursor movement. The depression of the pointing stick causes a decrease in resistance of the entire bridge. The static resistance across the bridge is nominally 3200 ohms. For the electronic circuits to sense a click or depression of the pointing stick, the z-axis output has to be maintained at a specified level. Typically, the resistance change when the pointing stick is depressed is very small. For example, in the z-axis resistors described in patent application Ser. No. 08/123456 the resistance changes 1.15 ohms for a 3200 ohm resistance across the bridge. This is 0.9 part per million per gram applied. This is a very small resistance change, 0.036% of the resistance value for the 3200 ohm resistance, x grams of force applied, for example, 400 grams.
In order to maintain a tolerance on the resistance change, a change in resistance ranging from 0.97 ohms to 1.32 ohms must be accurately measured. This is a range of 0.34 ohms. An 11.52 million error in measurement results in a error of 1 percent of the tolerance. Similarly, an error is introduced if the 400 gram weight is not accurate. When a voltage is applied to the resistor, the temperature of the resistor rises and the resistance can change. This problem must be considered in holding the tolerance during testing.
The small resistance change when the pointing stick is depressed and a tight tolerance in the allowable resistance range of the pointing stick results in difficulty in testing the z-axis resistors during manufacturing. The needed test system must be able to measure accurately and precisely and at the same time maintain the required test parameters. Further the test system must be able to quickly and rapidly be able to test large volumes of resistors. The test system also needs to be compact and low cost.